An IMS is a multimedia service communication network based on IP, and is regarded as a core technology that is of a next-generation communication network, supports services of a plurality of media, such as voice, video, and data, and their combinations, and implements integration of a plurality of networks (for example, mobile and fixed networks, Packet Switching (PS) and Circuit Switching (CS) networks). In an IMS network, an IMS core network subsystem is adopted for service control, and a packet switching network or a circuit switching network is used as an access network to provide a bearer signaling and media stream transmission, thereby separating a service control network from a bearer network, and providing a unified and flexible supporting platform for an IP multimedia application.
In the IMS network, a Call Session Control Function (CSCF), provides a core IMS session control function. The CSCF may be classified into a Proxy CSCF (P-CSCF), a Serving CSCF (S-CSCF), and an Interrogating CSCF (I-CSCF) according to its position and function. The P-CSCF is a first connection point with a user in the IMS, and provides a proxy function, namely, receives a service request and forwards the service request; the P-CSCF may also provide a user agent (UA) function, namely, interrupt and independently generate an Session Initial Protocol (SIP) session in an abnormal case. The S-CSCF is in a core control position on an IMS core network, is responsible for registration authentication and session control for a User Equipment (UE), executes basic session routing functions for IMS users at a calling end and a called end, and according to an IMS trigger rule subscribed by the users, performs value-added service route triggering to an Application Server (AS) and service control interaction with an AS when a condition is met. The I-CSCF is similar to a gateway node on the IMS, and provides functions such as allocation of user serving nodes in a local domain, route querying, and topology hiding between IMS domains.
An AS provides an IMS value-added service for an IMS user, and may perform service control through interaction with the S-CSCF.
The IMS adopts SIP signaling as call control signaling. SIP is an IMS control layer protocol, one of multimedia stream communication system framework protocols formulated by Internet Engineering Task Force (IETF), and an application layer protocol used to establish, change, or end a multimedia stream session, and cooperates with protocols such as Real Time Transport Protocol (RTP)/RTP Control Protocol (RTCP), Session Description Protocol (SDP), Real Time Streaming Protocol (RTSP), and Domain Name System (DNS) to jointly complete session establishment and media stream negotiation in the IMS. Once a session is established, a media stream is directly transmitted at a bearer layer by using the RTP protocol. A plurality of media streams may be flexibly interacted in one session.
In IMS Service Continuity (SC) session continuity when a user moves between different access networks is researched. That is, the user can change an access network without interrupting a session when moving. The core of the SC is an SCC AS, and the SCC AS completes a function of transferring between Access Leg in different access networks. An Access Leg is a call control path between a UE and an SCC AS. When it has been detected by an SC user equipment in a connection state that a change of a surrounding radio environment or other changes occur, the SC user equipment in the connection state performs access network transfer according to a predefined transfer policy.
A Single Radio terminal is a special type of terminal, where the terminal of this type can enable only one type of radio frequency at a certain moment. For example, when enabling a Universal Mobile Telecommunications System (UMTS) radio frequency, the UE cannot enable an Long Term Evolution (LTE) radio frequency. Transfer of a voice service of a single radio terminal between different access networks is referred to as Single Radio Voice Call Continuity (SRVCC), and transfer of a video service between different access networks is referred to as Single Radio Video Call Continuity (vSRVCC).
SCUDIF is a CS network service feature, a full name of which is Service Change and UDI Fallback feature. Unstructured Data Information (UDI) is a type of bearer capability, which enables a UE to send data at a rate of 64 kbit/s, and a typical application is a video call. A service change enables the UE to switch between a voice call and a video call during an ongoing call process without interrupting the call.
In the prior art, a major solution for transferring a video call from an IMS domain to a CS domain is to transfer voice media to the CS domain first, and then establish a new video call in the CS domain to replace the transferred voice media. In a handover process, a UE needs to first change the video call into a voice call, and then change the voice call back to a video call. Generally, the UE needs a certain period of time (for example, 5 to 8 seconds) for negotiating a video encoding manner in the CS domain. Therefore, the UE and a peer end undergo video interruption for at least a certain period of time. During the handover process, the voice media of the UE is held until CS video negotiation is completed. That is, voice communication of the UE also undergoes interruption for a certain period of time. Therefore, this solution results in communication interruption of voice and video for a certain period of time, which reduces user experience.